AbstractBoaden, P. J. S. (Queen“s University Marine Biology Station, Portaferry BT22 IPF, Northern Ireland.)Anaerobiosis, meiofauna and early metazoan evolution.Zool. Scr. 4 (1): 21–24, 1975.–Anaerobic marine sands have a fairly diverse fauna and flora consisting mainly of prokaryotes, protists, gnathostomulids, turbellarians, nematodes and gastrotrichs. An increasing body of evidence shows that the Metazoa from this environment are primitive in many respects. This paper suggests that much early evolution occurred in this habitat. Possible supporting evidence for this theory is discussed. It is concluded that the early Metazoa were benthic anaerobes and well established by the Middle Precambrian Era and that, in addition to the Prokaryota and Protista, the Gnathosto‐mulida, Platyhelminthes and Aschelminthes originally evolved under anaerobic con